Low-cost, electric, linear actuators are used in a variety of consumer products, including home appliances and automobiles, to move various components, including lock bolts, valve plates and the like, on the occurrence of an electrical signal.
Common linear actuators include solenoids, wax motors, and DC motors driving gear trains or screw threads. In a solenoid, a metal plunger loosely surrounded by a coil of wire is moved under the influence of a magnetic field produced by an electrical current in the coil. A wax motor employs an electrical current to heat wax contained in a closed volume so that the expanding wax drives a piston out of the volume.
Conventional solenoids and wax motors use a return spring to return the plunger or piston to its unactuated state, and thus require continued power to retain their actuated state. In contrast, small DC (direct current) motors, driving a rack-and-pinion gear or screw and nut, can be reversed by changing the polarity of the driving current, avoiding the need for a return spring and allowing the actuator to retain its actuated state after power is withdrawn.
One problem with DC motor linear actuators is friction in the gear train or screw and nut, particularly when the latter become contaminated during use. The high mechanical advantage typically present in a screw and nut design can cause jamming of the screw and nut at the end of travel under the momentum of the motor.